The invention relates to wireless communication systems in general, and more particularly to TDD communication using an antenna with more than one antenna element.
In CDMA (Code Division Multiple Access) a user""s narrowband data signal is modulated to a relatively wide band by means of a spreading code of a broader band than the data signal. Prior art CDMA systems utilize bandwidths of more than 1 MHz. In a WCDMA (Wide-band CDMA) radio system bandwidth is considerably broader still because current mobile networks wish to provide users with increasingly diversified services.
In the CDMA radio system a spreading code usually comprises a long pseudo-random bit sequence. The bit rate in the spreading code is much higher than in a data signal and to distinct spreading code bits from data bits and data symbols, they are called chips. Each data symbol of a user is multiplied by the spreading code chips, thereby spreading the narrowband data signal to the frequency used by the spreading code. Each user- has a specific spreading code, and it may be one or more data bits long. A plurality of users transmit simultaneously at the same frequency, and the data signals are separated in the receivers on the basis of the pseudo-random spreading code.
Data transmission in radio systems takes place in duplex mode which can be implemented by applying a FDD (Frequency Division Duplex) or a TDD (Time Division Duplex) principle. In the FDD solution reception and transmission are carried out at different frequencies. In the TDD solution reception and transmission are carried out at different instants of time, but the frequencies of the received signal and the transmitted one are the same.
At a base station of the SDMA (Spatial Division Multiple Access) radio system using the CDMA TDD technique in particular, a signal is received and transmitted through an antenna group comprising several antenna elements. Each antenna element is typically connected to one transceiver. Signals transmitted and received in a common baseband unit are phased in relation to each other to provide a radiation pattern of a desired form with the antenna group. A typical antenna pattern comprises a narrow main beam and a plurality of side beams. The direction and width of the main beam can be controlled by phasing the radio frequency signal~of each element. In practice the phasing is carried out by multiplying the digitised baseband antenna signal of each antenna element by complex coefficients shaping the antenna pattern. This solution is described in greater detail in WO 98/27669, included herein as a reference. According to the solution disclosed in WO 98/27669 the response of the antenna and hence the beam forming coefficients result in a matrix in a situation where the signals experience multipath propagation. To use the matrix in transmission leads to many problems such as: complicated receiver structure, unnecessarily heavy loss of orthogonality of the multipath signals due to a large number of paths, and extensive path losses.
An object of the invention is therefore to provide a method and an equipment implementing the method so as to allow the above problems to be solved. The invention relates to a method for forming a beam of a signal to be transmitted from at least one transceiver in a TDD communication system, the method comprising: receiving and transmitting a signal by using an antenna comprising at least two elements; receiving a multipath propagated signal from at least one different transceiver and estimating responses of the antenna at different delays for each different transceiver; selecting a subset of at least one response of the antenna for at least one different transceiver at such a delay or delays that the selected at least one response of the antenna has a local energy maximum; calculating beam forming coefficients for each different transceiver by using the selected subset; and multiplying the signal to be transmitted by the beam forming coefficients for each antenna element.
The invention also relates to a method for forming a beam of a signal to be transmitted from at least one transceiver in a TDD communication system, the method comprising: a base station receiving and transmitting by using an antenna comprising at least two elements; the base station calculating an estimate of received interference power; the base station transmitting a signal comprising information on the transmit power and the interference power at the base station; a subscriber equipment receiving the signal, forming a channel estimate and calculating the power of a signal to be transmitted to the base station on the basis of the information in the received signal and the channel estimate of the received signal; the subscriber equipment transmitting to the base station by using the estimated transmit power; the base station receiving the signal transmitted by the subscriber equipment and the base station calculating beam forming coefficients for each antenna element on the basis of the received signal by estimating responses of the antenna of the received signal at different delays; selecting a subset of at least one response of the antenna at such a delay or delays that the selected at least one response of the antenna has a local energy maximum; calculating the beam forming coefficients by using the selected subset; and multiplying the signal to be transmitted by the beam forming coefficients for each antenna element.
The invention further relates to a method for forming a beam of a signal to be transmitted from at least one transceiver in a TDD communication system in conjunction with a call set-up, the method comprising: a base station receiving and transmitting by using an antenna comprising at least two elements; the base station calculating an estimate of received interference power; the base station transmitting a common downlink signal comprising information on the transmit power and the interference power at the base station; a subscriber equipment receiving the common downlink signal, forming a channel estimate and calculating the power of an access signal to be transmitted to the base station on the basis of the information in the received common downlink signal and the channel estimate of the received common downlink signal; the subscriber equipment transmitting the access signal to the base station by using the estimated transmit power; the base station receiving the access signal transmitted by the subscriber equipment and the base station calculates beam forming coefficients for each antenna element on the basis of the received access signal by estimating responses of the antenna at different delays for the subscriber equipment; selecting a subset of at least one response of the antenna at such a delay or delays that the selected at least one response of the antenna has a local energy maximum; calculating the beam forming coefficients for each different transceiver by using the selected subset; and multiplying the signal to be transmitted by the beam forming coefficients.
The invention additionally relates to an arrangement for forming a beam of a signal to be transmitted from at least one transceiver in a TDD communication system that comprises at least one base station and at least one subscriber equipment which are transceivers, a transceiver comprising an antenna with at least two elements for receiving and transmitting a signal; the transceiver being arranged to receive a multipath propagated signal from at least one different transceiver and to estimate responses of the antenna at different delays for each different transceiver; the transceiver being arranged to select a subset of at least one response of the antenna for at least one different transceiver at such a delay or delays that the selected at least one response of the antenna has a local energy maximum; the transceiver being arranged to calculate beam forming coefficients for each different transceiver by using the selected subset; and the transceiver being arranged to multiply the signal to be transmitted by the beam forming coefficients for each antenna element.
The invention further relates to an arrangement for forming a beam of a signal to be transmitted from at least one transceiver in a TDD communication system that comprises at least one base station and at least one subscriber equipment which are transceivers, a base station comprising an antenna with at least two elements for receiving and transmitting signals; the base station being arranged to calculate an estimate of received interference power; the base station being arranged to transmit a signal comprising information on the transmit power and the interference power at the base station; a subscriber equipment being arranged to receive the signal, to form a channel estimate and to calculate the power of a signal to be transmitted to the base station on the basis of the information in the received signal and the channel estimate of the received signal; the subscriber equipment being arranged to transmit to the base station by using the estimated transmit power; the base station being arranged to receive the signal transmitted by the subscriber equipment and the base station being arranged to calculate beam forming coefficients for each antenna element on the basis of the received signal so that the base station is arranged to estimate responses of the antenna of the received signal at different delays; the base station is arranged to select a subset of at least one response of the antenna at such a delay or delays that the selected at least one response of the antenna has a local energy maximum; the base station is arranged to calculate the beam forming coefficients by using the selected subset; and the base station is arranged to multiply the signal to be transmitted by the beam forming coefficients for each antenna element.
The invention further relates to an arrangement for forming a beam of a signal to be transmitted from at least on transceiver during a call set-up in a TDD communication system that comprises at least one base station and at least one subscriber equipment which are transceivers, a base station comprising an antenna with at least two elements for receiving and transmitting; the base station being arranged to calculate an estimate of received interference power; the base station being arranged to transmit a common downlink signal comprising information on the transmit power and the interference power at the base station; a subscriber equipment being arranged to receive the common downlink signal, to form a channel estimate and to calculate the power of an access signal to be transmitted to the base station on the basis of the information in the received common downlink signal and the channel estimate of the received common downlink signal; the subscriber equipment being arranged to transmit the access signal to the base station by using the estimated transmit power; the base station being arranged to receive the access signal transmitted by the subscriber equipment and the base station being arranged to calculate beam forming coefficients for each antenna element on the basis of the received access signal so that the base station is arranged to estimate responses of the antenna of the received signal at different delays; the base station is arranged to select a subset of at least one response of the antenna at such a delay or delays that the selected at least one response of the antenna has a local energy maximum; the base station is arranged to calculate the beam forming coefficients by using the selected subset; and the base station is arranged to multiply the signal to be transmitted by the beam forming coefficients for each antenna element.
Several advantages are gained by the method and arrangement of the invention. The operation of the subscriber equipment can be simplified. As the number of paths can be restricted during transmission, the structure of the receiver can be made simpler, which usually means fewer correlators. Also the orthogonality of the multipath components can be better maintained because the number of paths is limited. The beam or the beams of the transmitter can be made to depend on the multipath propagation of the signal and/or the structure of the receiver, and path losses can be reduced.